Narrowing Your Piston Choices

You're at the point in your project where it's time to start building the engine for your car. You're looking online to see what pistons and rebuild kits are available, but the first thing you find out is that there are a lot of choices available when it comes to pistons. The decision can be a daunting one, and that's probably why we get a lot of readers emailing us, asking what pistons they should use in their engine. Since you guys asked, we decided to put together this article about piston selection, and how to choose what's right for your individual application. There is no way for us to determine what piston you think will be best, but one thing to keep in mind, is that the piston you choose is subjected to violent explosions, extremely high temperatures, and if your engine isn't running correctly, situations that are akin to your piston being hit by a sledge hammer.

Automotive pistons are an extraordinary piece of design. Within a matter of seconds, they are subjected to temperatures reaching upwards of 1,000 degrees during combustion, only to be immediately subjected to a blast of cold air with every intake stroke. They reach speeds of around 7,000 rpm, and have to withstand side loads that try to push it through the cylinder wall. To say that a piston is abused, is an understatement. So, how do they survive? If used in an improper application, they don't. Take for example, if you install a stock, cast piston in an application that will see higher cylinder pressures than stock because of a turbo, supercharger, or nitrous. You can rest assured, that you will be seeing your engine's internals become external. But, why is that? To fully understand, we need to explain what the different pistons are, and when is the right time to use which one.

Determine Power Goals

One important factor to consider when choosing a piston is balancing on the fine line between strength and price. Before you can decide what type of piston your engine needs, you need to know how much power you are planning to make before you purchase pistons. Obviously, the more power you build into your engine, the more expensive the required piston will cost. The way you decide to make that power is also a huge factor. Will you be using nitrous oxide? Maybe a blower is in your future. These are all aspects that need to be considered.

Cast vs. Hypereutectic vs. Forged

2/7A cast piston is built by pouring melted aluminum alloy into a mold that when cooled after filling, results in a piston-like “blank” being made.

The two most popular ways that a piston is manufactured, either results in a cast or forged piston. We've all heard these nomenclatures applied to pistons, but what does it mean—what's the difference? The difference is found in the way that the piston is actually made. A cast piston is built just like it's named. During the casting process, the melted aluminum alloy is poured into a mold that when cooled after filling, results in a piston-like "blank" being made. Casting a piston has a few advantages over forging when manufacturing. Cast tooling is generally designed to produce a near net casting. This near net shape minimizes the overall finish-machining that is required, reducing its cost. Cast pistons also offer excellent wear and thermal characteristics. This enables long ring-land and skirt life, as well as the ability to retain tighter side wall-clearances for quiet operation. The major drawback to running cast pistons is that the cast aluminum is limited in its ductility. Ductility is a solid material's ability to deform under tensile stress. What this means is that an over-stressed cast piston can fail more quickly.

When it comes to choices in aftermarket cast pistons, the choices begin with what we'll refer to as low-cost replacement pistons. These pistons are considered to be direct replacements, and are usually balanced closely to the weight of stock pistons. This is done so that "in theory," no balancing is needed when rebuilding. If you're building a performance-based engine, these pistons are not for you. With OE replacement style pistons, you can forget about features like valve notches for clearance to run higher-than-stock cams. It will also be difficult to get an adequate compression height for building any reasonable compression ratio. If building with any hopes of getting a high-performance engine, these pistons are almost always a risky purchase. But, if you're building a low rpm, daily-driver type of engine, these might be just what you're looking for.

An option that many consider slightly better for performance applications than cast pistons are hypereutectics. The word hypereutectic gets its name from the aluminum alloy used during the manufacturing process. While cast piston-alloys use silicon, which increases the wear resistance, durability, and thermal characteristics of the aluminum, there is a limit to how much silicon can be utilized so it is evenly distributed without large nodes of silicon forming. This level, which is called the eutectic point, is around 12 percent. Hypereutectic means pushing this level of silicon beyond the normal amount that can be simply blended into the metal. Hypereutectic aluminum will typically have between 16-18 percent silicon in the alloy. Although hypereutectic pistons are still a cast piston, the addition of this extra silicon material gives several benefits over conventional cast pistons. For starters, the piston is inherently stronger, with increased thermal characteristics, lubricity, and scuff resistance. It is also more resistant to corrosion, has more controlled expansion characteristics, and better high-temperature strength. While they are stronger than a typical cast piston, when pushed beyond their survivable limits, they will break similarly to a conventional cast piston. Cast and hypereutectic pistons are ideal for engines that witness a lot of street use with just the very occasional weekend dragstrip visit. If adding a power adder like nitrous, turbo, or supercharger, we recommend avoiding the use of a cast or hypereutectic piston.

3/7Cast pistons offer excellent wear and thermal characteristics which enables long ring-land and skirt life, as well as the ability to retain tighter side wall clearances for quiet operation. A drawback to running cast pistons is that the cast aluminum has limited ductility. This means that an over-stressed cast piston can fail more quickly.

Forged pistons are always considered an upgrade to cast and hypereutectic pistons. They inherently have metallurgical characteristics that differ from their cast counter parts. The reason for this is because instead of being melted and poured into a mold, blanks for forged pistons are created from a billet of extruded aluminum alloy. This billet is subjected to very extreme pressure, as the alloy is compressed into heavy forging dies that create the rough shape of the piston—the piston blank. This forging process results in a denser, more ductile piston. But, since the die is made from two mirrored halves, this eliminates the possibility of designing the internal area in the raw forging. Because of this, a forged-piston blank needs a lot more machining to create the actual piston. But, this denser, more ductile material means that a forged piston is inherently stronger and more forgiving when its limits are exceeded.

Another decision to consider is that there are different alloys used in forged pistons. The most common alloys are 2618 (a low-silicon aluminum), and 4032 (an alloy with around 11-percent silicon). The 4032 alloy is a stiffer material, and has less thermal conductivity and rate of expansion. It also contains a great resistance to wear. 4032 is typically the alloy of choice in street/strip applications. It's selected because of the capability of tighter piston-to-wall clearance gained by the lower expansion rate. Another benefit is longer skirt and ring-belt life offered by the greater wear resistance of the higher-silicon alloy.

The 2618 alloy has a low-silicon content. While it does give up some of the benefits achieved from silicon, it is commonly agreed to be the best piston material for brutal all-out race applications. The lower-silicon alloy has a higher density rating and tensile strength than 4032, and has a much greater ductility and fracture resistance. A 2618-alloy piston can be beaten flat, before breaking. The high rate of thermal conductivity of 2618 alloy will allow the transfer of more heat into the skirts. Keep in mind, this material's higher expansion rate generally means significantly more bore clearance (piston-to-sidewall). The 2618 piston is more subject to wear in the ring lands and skirts. 2618 alloy pistons are designed to be racing pistons. Don't install these in your car or truck, and expect to travel 100,000-plus miles. If the plan calls for serious nitrous, boost, or rpm, and bulletproof is what you are after, a 2618 piston is the right choice. Current machining and design technologies have resulted in more sophisticated skirt designs. Manufacturers taking advantage of these advances have been able to reduce the bore clearance considerably compared to traditional requirements.

4/7Before selecting your pistons, you will have to know the required compression height so you can get to the actual compression ratio for the engine. Compression height is the distance between the centerline of the wrist pin bore and the top of the piston. When using a flat top piston, this can be simple math. But, add valve notches or a “pop up” dome, and now you need to do some math. The dome volume is comprised of the dome (material above the piston’s deck), minus the valve notches. This final number will either be a negative of positive. If the volume of the dome is greater than the volume of the notches (typical), then you have a positive dome height.

Piston-to-Cylinder Wall Clearance

No matter which piston you choose, piston-to-cylinder wall clearance is a major factor in fitting the pistons to the cylinders. The working characteristics of both forged and cast pistons do vary considerably with the specific material and processes involved in their manufacture. What this means is that not all cast pistons are created equal. The same can be said of forged pistons. When deciding whether a certain type of piston is suitable for a specific application, it pays to talk to the manufacturer. Forged pistons are suggested whenever the revs are expected to be high, compression ratio is increased, and power adders will be utilized. Different skirt thicknesses will expand at different rates, even within the same piston design. Other aspects that will affect the piston's expansion rate are the piston skirt length, ring belt (area between the top of the piston and the wrist pin) thickness, and engine cooling characteristics. All manufactures give a nominal piston to wall clearance that they want you to go by. In most cases, the manufacturer will finish-machines the piston's diameter to include the clearance for the given bore diameter. A 4.030-inch bore piston would come out of the box with a 4.026-inch diameter if the recommended clearance was .004-inch.

When looking at alloys and their thermal expansion rates, remember that all metals have thresholds. While all pistons will expand at a given rate, there are also achievable temperatures they can reach that will cause them to not follow the given "standard" rate of expansion. Pistons will react differently because of varying mass, thicknesses, and heat exposure. Although not perceptible to the naked eye, most pistons do not have a "flat" side wall. Pistons have what is called a cam shape or profile. The cam shape is design of the skirt that makes it oblong where it rides on the cylinder wall. If you have a longer skirt, it will make your cam shape longer or shorter depending on the scale that is used. For this reason, it is imperative that the manufacturer's suggestion on clearances be adhered to.

Piston Coatings

Piston coatings can help curb the problems of wear, heat, friction, and corrosion. Some performance coatings include ceramic thermal barriers, dry film lubricants, ceramic exhaust coatings, oil shedding coatings, and several other coatings to improve the performance and/or durability of high performance parts.

5/7Forged pistons are always considered an upgrade to cast and hypereutectic pistons. They are created from a billet of extruded aluminum alloy. This billet is compressed into heavy forging dies that create the rough shape of the piston—the piston blank. But, since the die is made from two mirrored halves, this eliminates the possibility of designing the internal area in the raw forging. This denser, more ductile material means that a forged piston is inherently stronger and more forgiving when its limits are exceeded. There are also different alloys used in forged pistons. The most common alloys are 2618 (a low-silicon aluminum), and 4032 (an alloy with around 11-percent silicon).

Pistons can be coated with three different types of coatings: dry film lubricants, thermal barriers, and oil shedding coatings. Thermal barrier coatings help protect the piston top against damaging heat-transfer by minimizing the amount of heat that is retained on the surface of the piston. This coating also allows heat at the surface to move more evenly over the surface, reducing hot spots or evens reflecting heat into the chamber for more efficient combustion of the fuel and less thermal expansion due to a reduction in the heat absorbed. A dry-film coating is typically applied to the skirt of the piston. This dry film will help reduce friction, and also inhibits galling during initial break-in. An oil-shed coating can be applied to the underside of the piston. This coating is intended to repel oil quicker than an untreated part.

Determine Compression Ratio

You can't build an engine, and not take the compression ratio into account. You will next have to determine what compression pistons will best accommodate your goals. For this section, let's remove the cast vs. hypereutectic vs. forged aspect, and just focus on the usage. A general rule of thumb is that you can run a higher compression ratio for all-motor, normally aspirated applications, and if adding power adders like a turbo or supercharger, you need a lower compression ratio.

Popular compression ratios for normally aspirated cars range from 9.5:1 and up. Again, a higher compression ratio will make a more responsive and powerful engine, but is also more prone to detonation issues if the tune is not accurate. Also, higher octane gas is usually required when going to higher compression pistons to deter detonation. Popular compression ratios for turbo or supercharged applications range from 8.0:1 to 9.5:1. The higher the compression ratio, the better your tune needs to be to keep the engine from detonating.

Before selecting your pistons, you have to know the desired compression height of said piston, so you can get to the actual compression ratio for the engine. The compression height is the distance between the centerline of the wrist pin bore and the top of the piston. When using a flat top piston, this can be simple math. But, add valve notches or a "pop up" dome, and now you need to use your noggin. The piston's dome volume is typically something that the manufacturer will publish, but if for some reason, you're using swap-meet sourced pistons, you'll need to figure things out for yourself. To do that, the volume is comprised of the dome (material above the piston's deck), minus the valve notches. This final number will either be a negative of positive. If the volume of the dome is greater than the volume of the notches (typical), then you have a positive dome height. Domed pistons will lessen the combustion chamber's volume since the dome takes up space above the piston's deck, and into the cylinder head chamber. With flat-top pistons, the volume considered is just the valve notch clearance—if so notched. If the pistons have been massaged with custom cutting or profiling on the domes, about the only way to accurately get the cc volume is to measure the domes directly.

6/7Piston coatings can help curb the problems of wear, heat, friction, and corrosion. Some performance coatings include ceramic thermal barriers, dry film lubricants, ceramic exhaust coatings, oil shedding coatings, and several other coatings to improve the performance and/or durability of high performance parts.

Speed Reading Version: Piston Material Comparisons

Cast Aluminum Pistons

Best suited for stock engines.

Lower price point.

The typical cast piston is melted, and then poured into a mold having the shape of the finished product. Piston molds are permanent dies, intricately made of multiple-piece steel shapes. The resulting casting requires minimal machining. Cast pistons cost less, and as such, are designed as a cost effective replacement option. For this reason, they are not well suited for high performance applications.

Hypereutectic (Cast) Aluminum Pistons

Best suited for up to roughly 600-650 horsepower in normally aspirated engines.

Middle of the road price point.

The hypereutectic piston is also a cast piston, but with added silicon (approximately 16 percent), to produce a harder, maore wear resistant version of the standard cast piston. Silicon itself expands less than aluminum, because it also acts as an insulator, preventing the aluminum from absorbing as much of the operational heat as a standard cast piston. Another benefit of adding silicon is that the piston becomes harder and is less susceptible to scuffing. Also, the higher silicon content of the hypereutectic pistons allows closer piston-to-sidewall clearances, improving combustion seal due to reduced rocking of the piston as it travels in the cylinder.

Because of the higher silicon content, hypereutectic is a less ductile alloy and is less forgiving when used with boosted and/or nitrous applications, making them best suited for normally aspirated engines.

4032 Forged Aluminum Pistons

Best suited for engines producing up to 1,000 horsepower normally aspirated.

Higher price point.

Forged pistons are the strongest on the market. The manufacturing process is different from a cast piston, because the aluminum is not molten like that of a cast piston. Instead, a hot aluminum ingot is forced into the simple mold. The result is a piston blank that needs more final machining than a cast piston before it becomes a piston. Forged 4032 alloy is a high-silicon (approximately 11 percent), alloy with low expansion characteristics. Pistons made from this alloy can utilize tighter piston-to-wall clearance, resulting in a tighter seal with less noise and reduced scuffing than its 2618 cousin. This closer tolerance keeps the piston from rocking, creating a more stable piston.

Because of the higher silicon content, 4032 is a less ductile alloy. This means that it is less forgiving when used with boosted and/or nitrous applications. They have a lower resistance to detonation, making them best suited for engines utilizing minimal amounts of boost, or normally aspirated engines.

7/7The most common forged alloys are 2618 (a low-silicon aluminum), and 4032 (an alloy with around 11-percent silicon). 4032 is typically the alloy of choice in street/strip applications. It’s selected because of the capability of tighter piston-to-wall clearance gained by the lower expansion rate. Another benefit is longer skirt and ring-belt life offered by the greater wear resistance of the higher-silicon alloy. The 2618 alloy does give up some of the benefits achieved from silicon, it is commonly agreed to be the best piston material for brutal all-out race applications. The lower-silicon alloy has a higher density rating and tensile strength than 4032, and has a much greater ductility and fracture resistance. 2618 alloy pistons are designed to be racing pistons. Don’t install these in your car or truck, and expect to travel 100,000-plus miles. If the plan calls for serious nitrous, boost, or rpm, and bulletproof is what you are after, a 2618 piston is the right choice.

2618 Forged Aluminum Pistons

Best suited for engines producing up to 1,200 horsepower, and ideal for engines running power adders.

Higher price point.

The 2618 aluminum material contains a low-silicon (approximately 2 percent) content. This low silicon creates high-expansion characteristics, and is the alloy typically used for extreme-duty racing applications. Due to its high-expansion characteristic, 2618 pistons are engineered to use larger piston-to-cylinder wall clearance. What this means, is that when you start the engine when cold, the pistons can be heard, and this is commonly referred to as the "piston slap." Once the engine warms up, the "slapping" noise quiets as the piston expands to its normal running clearance.

2618 is a more ductile alloy, and this feature gives a higher resistance to detonation. These characteristics enable the piston to survive some of the most extreme conditions, but longevity is eventually compromised after countless heat cycles.